Ukrainian President Petro Poroshenko has declared a week-long ceasefire in the fight against pro-Russian separatists in the east of the country.

But the Kremlin dismissed the move as an ultimatum, as the US said it had evidence Moscow was stockpiling tanks and artillery close to the border with its neighbour, which could be supplied to rebel fighters.

Mr Poroschenko said the declaration of a ceasefire would allow him to formally launch a peace plan for the region, which has been blighted by deadly violence.

Mr Poroshenko warned his forces would defend themselves if attacked

He has said it will include amnesty for pro-Russia fighters who have not committed serious crimes, a corridor for fighters from Russia to leave the country, joint security patrols, early local and parliamentary elections, and protections for the use of the Russian language.

But he warned government forces would defend themselves if they came under attack from the rebels.

"We will do everything to defend the territory of our state," said Poroshenko during a visit to the troubled Donetsk region.

But in a statement, the Kremlin said: "This is not an invitation to peace and negotiations but an ultimatum to militias in the southeast of Ukraine to lay down their arms."

Foreign Secretary William Hague has urged Russia to support the peace plan and called on pro-Russia separatists to lay down their arms.

Against the backdrop of continuing tensions, the US said it had information of a Russian build-up of military hardware close to the frontier with Ukraine that could be provided to separatists.

Most of the equipment being stockpiled is of the type still used by Ukrainian forces, which an official said left "the impression that the desire here is to mask the Russian hand and allow the separatists to assert ... that this is material that they've captured on the battlefield from Ukrainians".

Washington also said it had evidence Russia had redeployed several thousand troops to the frontier.

The official said some of the forces were "within a handful of kilometres" of Ukrainian territory, "the closest they have been since the invasion of Crimea" earlier this year.

But the Kremlin said the forces were simply implementing earlier orders of President Vladimir Putin's to "reinforce the protection of the Russian border".

The latest developments in the Cold War-style stand-off came as the US Treasury Department blacklisted seven Ukraine separatists, saying their activities threaten the peace and sovereignty of the country.

The West has warned Moscow of "more costs" if it does not work to de-escalate the situation.